1. Technical Field
The present invention relates to a liquid crystal display panel, and more particularly to a liquid crystal display panel and a fabricating method thereof that are capable of increasing the response speed of a liquid crystal to improve display quality of the display panel.
2. Description of the Related Art
Generally, a liquid crystal display (hereinafter, referred to as “LCD”) device controls the light transmittance of liquid crystal cells in response to a video signal to display a picture corresponding to the video signal on a liquid crystal display panel in which the liquid crystal cells are arranged in a matrix type.
Liquid crystal display devices are generally either classified as vertical electric field applying type devices or horizontal electric field applying type devices depending upon with a direction of an electric field that drives the liquid crystal.
The vertical electric field type of liquid crystal display device forms a pixel electrode and a common electrode arranged in opposition to each other on the upper and lower substrate, and applies an electric field in a vertical direction with the liquid crystal cell using a voltage applied to the electrodes. The vertical type liquid crystal display device normally has a large aperture ratio, although it normally has a narrow viewing angle. A representative liquid crystal mode of vertical electric field applying type display device shown in the sectional view of FIG. 1 is shown with a twisted nematic mode (hereinafter, referred to as “TN”) provided at a liquid crystal display terminal.
The liquid crystal display panel includes an upper array substrate 60 (or a color filter array substrate) comprised of a black matrix 4, a color filter 6, a common electrode 18 and an upper alignment film 8 which are sequentially provided on an upper substrate 2. A thin film transistor 20 (hereinafter, referred to as “TFT”) is provided on a lower substrate 32, a lower array substrate 70 (or a thin film transistor array substrate) that includes a pixel electrode 16 and a lower alignment film 38. Liquid crystals 82 are injected into an internal space between the upper array substrate 60 and the lower array substrate 70.
In the upper array substrate 60, the black matrix 4 corresponds to the TFT 20 area of a lower plate, gate line and data line areas are formed on the upper substrate 2, and define a cell area to be provided the color filter 6. The black matrix 4 prevents light leakage and absorbs an external light to improve a contrast ratio. The color filter 6 is formed at a cell area divided by the black matrix 4. The color filter 6 is provided for each R, G and B (corresponding to red, green, and blue, respectively) to provide the red, green, and blue colors on the display. The common electrode 18 is formed at a front side of the upper substrate 2 and is supplied with a common voltage that serves as a reference voltage for driving the liquid crystal. A spacer 13 maintains a cell gap between the upper array substrate 60 and the lower array substrate 70.
In the lower array substrate 70, the TFT includes a gate electrode 9 provided on the lower substrate 2 along with a gate line, semiconductor layers 14 and 47 overlap the gate electrode 9 with a gate insulating film 44 therebetween. Source/drain electrodes 40, 42 are formed along with a data line (not shown) with having the semiconductor layers 14 and 47 therebetween. Such a thin film transistor supplies a pixel signal from the data line to the pixel electrode 16 in response to a scanning signal from the gate line.
The pixel electrode 16 is a transparent conductive material having a high light transmittance and contacts the drain electrode 42 of the TFT 20 with a protective film 50 therebetween. An alignment material, such as a polyimide, is coated and then a rubbing process is carried out to provide an upper/lower alignment film 8, 38 for aligning of the liquid crystal.
Hereinafter, an operation of the TN mode on the occasion of a normally white mode will be described with reference to FIGS. 2A and 2B. An upper polarizer 72 with a light transmission axis oriented in a specific direction is attached at a light emitting surface of the upper array substrate 60, and a lower polarizer 62 having a light transmission axis perpendicular to the light transmission axis of the upper polarizer 72 is attached on a light incidence surface of the lower array substrate 70.
Referring specifically to FIG. 2A, a local optical axis (a director) of the liquid crystal 82 molecules is continuously twisted in such a manner to have a value of 90° between the upper array substrate 60 and the lower array substrate 70 on an off-state that a voltage is not applied to the upper array substrate 60 and the lower array substrate 70. The characteristics of a linearly polarized light passing through the lower polarizer 72 are modified to pass through the upper polarizer 62 at the off-state. Referring to FIG. 2B, in the TN mode an optical axis of a middle portion of a liquid crystal layer 80 is in parallel to the electric field at an on-state, where voltage is applied to the common electrode 18 and the pixel electrode 16 and the electric field is applied to the liquid crystal layer 80 by a voltage difference thereof to disentangle a twisted structure of the liquid crystal layer 80. A polarized incident light passing through the lower polarizer 72 additionally passes through the liquid crystal layer 80 at the on-state to maintain the polarized light characteristics as it is. The polarized light does not pass through the upper polarizer 62.
On the other hand, in the TN mode having the above-mentioned operation characteristics, a major axis of the liquid crystals 82 are arranged in a horizontal direction on the upper and lower alignment film 8 and 38 at the off-state, so that a pretilt angle is less than 5°. Accordingly, since the pretilt angle is so low at the off-state, then a speed that an optical axis of the liquid crystal 82 molecules are entirely in parallel to the electric field at the on-state, that is, a response speed, is slowed down. Although an initial alignment generally has a high pretilt angle with a vertical alignment film, a time that the liquid crystal 82 aligned in a vertical direction is arranged in a horizontal direction, is also slowed down. Upon implementing an image, a residual image is generated on the screen by a response speed delay of such a liquid crystal, so that a display quality is reduced.